1. Technical Field
The present invention relates to a gas sensor element that includes a basal body having a bottomed tubular shape and made of an electrically insulative ceramic material, a solid electrolyte portion and a pair of electrodes, and to a method of manufacturing the gas sensor element.
2. Description of Related Art
There are known gas sensor elements that are employed in, for example, lambda sensors or A/F (Air/Fuel) ratio sensors to detect the concentration of oxygen in the exhaust gas from an internal combustion engine of a motor vehicle.
More specifically, lambda sensors are generally configured to detect the concentration of oxygen in the exhaust gas based on an electromotive force outputted from the gas sensor element; the electromotive force represents the difference in oxygen concentration between the exhaust gas and a reference gas (e.g., air). On the other hand, A/F ratio sensors are generally configured to detect the concentration of oxygen in the exhaust gas based on a limit current outputted from the gas sensor element; the limit current also represents the difference in oxygen concentration between the exhaust gas and the reference gas. In addition, the A/F ratio sensors are generally configured to further determine the A/F ratio of air-fuel mixture supplied to the engine based on the detected concentration of oxygen in the exhaust gas.
Moreover, the known gas sensor elements are generally configured to include a solid electrolyte body and a pair of measurement and reference electrodes. The solid electrolyte body is made, for example, of zirconia partially stabilized by yttria. The measurement and reference electrodes are made of, for example, platinum and respectively provided on opposite surfaces of the solid electrolyte body so as to be respectively exposed to the exhaust gas (i.e., the measurement gas or the gas to be measured) and the air (i.e., the reference gas). In addition, on the measurement electrode, there is further provided a protective layer in those gas sensor elements which are used in lambda sensors and a diffusion-resistant layer in those gas sensor elements which are used in A/F ration sensors.
Furthermore, the known gas sensor elements generally have either a plate shape or a bottomed tubular shape.
More specifically, the plate-shaped gas sensor elements are generally formed by laminating a solid electrolyte layer and insulating layers. Therefore, it is easy to manufacture the plate-shaped gas sensor elements. Further, it is possible to laminate a heater layer together with the solid electrolyte and insulating layers, thereby easily heating the solid electrolyte layer in operation. However, due to the plate-like shape, corner portions are formed at ends of the gas sensor elements. Consequently, with the corner portions, the gas sensor elements may be easily damaged by, for example, thermal shock caused by water generated in the exhaust pipe.
On the other hand, for the bottomed tubular gas sensor elements, it is possible to configure a bottom surface thereof as a curved surface. Consequently, with the curved bottom surface, it is possible to alleviate thermal shock caused by water generated in the exhaust pipe, thereby preventing the gas sensor elements from being damaged by the water.
Moreover, to minimize the manufacturing cost of the gas sensor elements, it is desired to minimize the amount of solid electrolyte used in the gas sensor elements.
To meet the above desire, Japanese Unexamined Patent Application Publication No. H3-138559 discloses a gas sensor element which includes a hollow cylindrical heater body. In the outer surface of the heater body, there is formed a groove so as to communicate with the hollow space formed in the heater body via a through-hole. A solid electrolyte layer is formed on the outer surface of the heater body so as to cross over the opening of the groove.
With the above configuration, however, there is a difference in level (or height) between the outer surface of the heater body and the outer surface of the solid electrolyte layer. Therefore, during the firing process of the gas sensor element or when thermal shock is applied to the gas sensor element due to water contained in the exhaust gas, stress concentration may occur at the boundary between the heater body and the solid electrolyte layer due to the difference in surface level therebetween. Consequently, damage may be caused to the gas sensor element, such as causing cracks to occur in the heater body or in the solid electrolyte layer or causing electrode lead wires of the gas sensor element to be broken.